1. Field of the Invention
The present invention relates to a sampling frequency control method which is applied to, for example, a digital protective relay for protecting a power system from a system failure on the basis of sampled values obtained by sampling an AC quantity of electricity of the power system at a prescribed sampling frequency, as well as to a protective relay in which the sampling frequency is controlled by the above sampling frequency control method.
2. Description of the Related Art
In conventional digital protective relays, a sampling frequency control for varying the sampling period so that it follows the frequency of an input AC signal is performed in the following manners. JP-A-2-13220 (FIG. 1 and a related description) discloses a technique that the period of an input AC signal is calculated on the basis of intervals of zero-cross points where the polarity of the input signal changes and the sampling period is corrected on the basis of the calculated period. JP-A-3-230722 (FIG. 1 and a related description) discloses the following technique. In a means for determining a current frequency of a power system and correcting the sampling period according to the determined frequency, zero-cross points of a voltage, for example, are stored as frequency timing and a frequency is determined by comparing current frequency timing with frequency timing obtained a prescribed time before.
In conventional digital protective relays, for example, the sampling frequency is controlled so as to correspond to an electrical angle 30° for a system frequency by calculating the system frequency on the basis of zero-cross points where the polarity of an input AC voltage changes and controlling the sampling timing of the protective relay on the basis of the calculated system frequency. There is a problem that the control may be based on calculation results including errors because zero-cross points are determined through approximation. For example, zero-cross points are determined by performing an interpolation calculation (linear approximation) on digital data obtained by AD-converting an input AC quantity of electricity (JP-A-2-13220).